Hybrid vehicles generally use motor generators at slow engine speeds, at which the motor generators have better torque characteristics than internal combustion engines, and use internal combustion engines at moderate to fast speeds, at which the engines have better torque characteristics. This improves fuel efficiency, as the engine is not used when the vehicle travels at slow speeds.
To control a hybrid vehicle, two system efficiencies are calculated, and the vehicle is controlled with the higher of the calculated system efficiencies. The system efficiencies are calculated by Equations 1.
                                          η                          sys              ,              dchg                                =                                    P              demand                                                      P                fuel                            +                                                P                                      b                    ,                    out                    ,                    real                                                  /                                  η                  bd                                                                    ⁢                                  ⁢                              η                          sys              ,              chg                                =                                                    P                demand                            +                                                (                                      P                                          b                      ,                      in                      ,                      real                                                        )                                ⁢                                  (                                      η                    bc                                    )                                ⁢                                  (                                      η                    bd                                    )                                                                    P              fuel                                                          Equations        ⁢                                  ⁢        1            where:    ηsys,dchg denotes system efficiency of a driving state of discharging the battery,    ηsys,chg denotes system efficiency of a driving state of charging the battery,    ηbd denotes discharge efficiency of the battery,    ηbc denotes charge efficiency of the battery,    Pdemand denotes a required driving power,    Pfuel denotes power of the internal combustion engine,    Pb,out,real denotes real discharge power of the battery, and    Pb,in,real denotes real charge power of the battery.
FIG. 2 illustrates a simulation result of a test performed on a hybrid vehicle on the basis of the system efficiency calculated according to the above-described method. The initial state of charge (SOC) of the battery is 60%. After the simulation ends, the state of charge of the battery is 53.33%. Relative fuel efficiency is set to 1 as a reference.
This method does not consider energy loss that occurs when the battery is charged. Therefore, when the SOC of the battery is used in a range from 50 to 70%, system efficiency of discharge is always calculated as being higher, and discharging the battery 13 is favored, leading to the battery being discharged over time.